Liquid crystal display devices are being used in watches, calculators, various measurement instruments, automobile panels, word processors, electronic organizers, printers, computers, televisions, clocks, advertising boards, etc. Representative examples of liquid crystal display modes are twisted nematic (TN) mode, super twisted nematic (STN) mode, and vertical alignment (VA) or in-plane-switching (IPS) mode that uses thin film transistors (TFTs). The liquid crystal compositions used in these liquid crystal display devices are required to be stable against external factors such as moisture, air, heat, and light, exhibit a liquid crystal phase (nematic phase, smectic phase, or blue phase, for example) in a temperature range as wide as possible around room temperature, have low viscosity, and operate at low drive voltage. The liquid crystal compositions are composed of several to dozens of compounds in order to optimize dielectric anisotropy (Δ∈), refractive index anisotropy (Δn), etc., for each individual display device.
Horizontal alignment displays such as TN, STN, or IPS displays use liquid crystal compositions having positive Δ∈. There has been a report of a driving mode with which a liquid crystal composition having a positive Δ∈ is aligned vertically in the absence of voltage and images are displayed by applying a horizontal electric field. The need for liquid crystal compositions having positive Δ∈ has increased as ever. Meanwhile, improvements in response speed are required in all drive modes, and a liquid crystal composition having a viscosity lower than those of the existing products has been pursued to meet this need. In order to obtain a low-viscosity liquid crystal composition, it is effective to lower the viscosity of each polar compound constituting the liquid crystal composition. Increasing Δ∈ of polar compounds used can decrease the ratios of low-viscosity nonpolar compounds added and can thereby decrease the viscosity of the liquid crystal composition. In order to use the liquid crystal composition in a display device or the like, the liquid crystal composition is required to exhibit a stable nematic phase over a wide temperature range. In order to keep a nematic phase over a wide temperature range, each component of the liquid crystal composition is required to have high miscibility with other components.
In general, introducing a large number of polar atoms, such as fluorine atoms and oxygen atoms, is effective for yielding high Δ∈. However, it is known that merely increasing the number of polar groups introduced decreases miscibility with the liquid crystal composition and causes drawbacks such as precipitation. It is believed that, in order to obtain a low-viscosity compound, it is effective to use a compound having plural ring structures, such as 1,4-cyclohexylene groups and 1,4-phenylene groups, directly bonded to one another without any bonding groups, in other words, a directly-bonded-ring compound. However, directly-bonded-ring compounds generally have high crystallinity and poor miscibility with liquid crystal compositions. Compounds having various bonding groups introduced therein are being studied to overcome this problem. It has been made clear that introduction of bonding groups can improve miscibility of the compounds with the liquid crystal compositions (PTL 1 to PTL 8). The following compound is a compound disclosed as having a further lower viscosity and high miscibility with a liquid crystal composition (PTL 9); however, the Δ∈ thereof is not sufficiently large.
